A viral hemorrhagic septicemia (VHS), which is also called egtved disease, is a viral disease occurring primarily in salmonid fish species as well as in a Japanese flounder, a red sea bream, a black rockfish, and the like. The disease attributable to viral hemorrhagic septicemia virus (VHSV) as a pathogen has been known for a long time as a disease of fishes in a fresh water area system mainly in the European continent; however, in recent years, it has also been reported from fresh water areas in North America and sea water areas in Japan, and the water area in which the disease occurs is gradually expanding.
As symptoms of this disease, there are observed melanic body color, protrusion of the eyes, abdominal swelling, hemorrhage in the eyeball, the branchia, the side of the body, and the base of the fin, and behavioral abnormalities such as swimming inactivity and abnormal swimming. In autopsy findings, congestion, expansion, and fading of the liver, kidney, and spleen and petechial hemorrhage of the skeletal muscle are seen (Non Patent Literature 1).
The disease non-latently infects younger fish more easily and has a high mortality of the younger fish; thus, it can cause heavy damage to the fishing industry, particularly to the culture industry. Therefore, there is an urgent need for the development of a simple detection method of the disease as an important disease to the fishery, and a prophylactic and therapeutic method thereof (Non Patent Literature 2).
However, an effective control method against the disease is not yet known; nowadays, a method is only adopted which involves early detecting affected individuals under a health monitoring program and removing them from the population. But, the disease has a large difference in symptoms between individuals; there exist from individuals having dramatic symptoms and resulting in death to individuals appearing almost normal. As a result, individuals having become latent virus carriers viable despite being infected individuals have a high possibility of evading the monitoring. Thus, the control method includes a problem of expanding/spreading the infection to the surrounding water area via such carriers. It also has a problem that it requires a great deal of labor for the monitoring.
Meanwhile, Patent Literature 1 discloses the invention of a DNA vaccine against VHSV. The DNA vaccine is a DNA vaccine encoding an immunogenic polypeptide of VHSV and is excellent in that it can be administered to intended individuals to stimulate the protective immunity of the individuals against VHS to impart the ability to be protected from VHS to the individuals. Because of being DNA, the vaccine also has the advantage that it is stable under high temperature, can be stored for a long period of time, can be rapidly and easily improved by a genetic method, and enables a reduction in time necessary for the development of the vaccine. However, the DNA vaccine has the disadvantage that it is not immediately effective and its protective effect does not appear until about 2 weeks after the inoculation thereof. In addition, a transgenic fish having recombinant plasmid DNA encoding the antigen also has a problem in terms of safety as an edible fish.